The present crisis in livestock agriculture has prompted a resurgence of interest in grass-fed animals. However, while a high-forage diet may be desirable, it does not currently satisfy the demands of modern animal production. For the animal to make efficient use of the forage it consumes, the energy demands of the micro-organisms in the rumen must be met and synchronized with the availability of plant proteins. Otherwise this lack of synchrony will lead to (a) proteins and other nutrients being poorly utilized in the rumen, (b) loss of nitrogen, in urine and feces and therefore, the environment and (c) the need to feed excessive amounts of protein concentrates as supplements to the ruminant diet.
Cellulose and hemicellulose in grass and maize tissues could meet the energy requirements of the ruminant or provide new feed-stocks for industrial fermentation to ethanol. This potential is not currently realized because the cell walls are lignified and the cell wall polysaccharides highly cross-linked with phenolic residues and lignin, resulting in low rates of plant cell wall digestion in comparison to rates of protein breakdown in ruminants. This is a particular problem for the most important forages in Europe, the ryegrasses Lolium perenne and L. mutiflorum as well as one of the major impediments to the wider use of better adapted species, such as Festuca arundinacea, as a forage crop. Increasing the digestibility index of grasses has therefore been a major breeding objective for several decades but progress has been slow due to difficulties in fixing natural variation in the synthetic varieties derived from these outbreeding species (Hayward, et al., TAG 70:48 (1985)).
Removing labile phenolics by chemical treatment with alkali is known to increase the biodegradability and nutritional value of low-quality feed such as cereal straw, and is employed commercially for feed upgrading. Reducing phenolic cross-linking of cell wall carbohydrates is therefore a predictable way of improving the rate of digestion and digestibility of ryegrass. However chemical modification may have other disadvantages. Therefore, genetic modification would be a preferable method of changing the cell wall chemistry of highly digestible varieties. Many in the field are pursuing this approach. An alternative, however, is to use genetic modification to reduce the levels of phenolic acids in the cell walls available for crosslinking either by directly disrupting ester bonds linking phenolics and lignins to cell wall polysaccharides or by preventing excessive ferulation of cell wall carbohydrates prior to their incorporation into the cell wall.
This invention meets this and other needs by using targeted or inducible expression of cell wall degrading enzymes in plants.